Acids are characterized by their sour taste, their ability to dissolve metals, and their reaction with carbonate compounds to release carbon dioxide gas (CO₂).

From a scientific perspective, the concept of the acid has been explained over time through different definitions. According to the Arrhenius definition, acids are substances that donate hydrogen ions (H⁺) or protons in aqueous solutions. The Brønsted–Lowry approach treats acids as proton-donating species. The more general Lewis definition characterizes acids as substances that accept an electron pair. These viewpoints allow acids to be examined not only through sensory properties but also in terms of their behavior at the molecular and atomic levels.^【1】 

The word “acid” derives from the Latin acidus or acere, meaning “sour.” This etymology points to the readily recognized sour taste of substances such as lemon juice, vinegar, and acetic acid. Prior to the development of modern chemistry, such taste-based descriptions were long used as basic criteria.

Modern chemical research has shown that acids should be considered not by taste or odor alone but within the contexts of proton transfer and electron interactions. Thus, the concept has evolved from sensory descriptions to molecular definitions that can be tested scientifically.

General Properties of Acids

Acids constitute a class of substances distinguishable by both physical and chemical properties. These traits are important for interpreting results from laboratory experiments as well as the effects of acidic substances encountered in daily life.

Taste and Contact

A conspicuous feature of acids is their sour taste, evident in natural organic acids such as citric acid in lemon, acetic acid in vinegar, and malic acid in apples. However, tasting acids in a laboratory setting is highly dangerous and inappropriate. Strong acids (e.g., sulfuric and hydrochloric acids) are corrosive and irritating on direct skin contact, leading to tissue damage, degradation of organic materials, and corrosion of metal surfaces. Protective measures are therefore essential when working with acids.

Electrical Conductivity

When dissolved in water, acids ionize to release hydrogen ions (H⁺) or hydronium ions (H₃O⁺) along with their corresponding anions. These ions move freely in solution and conduct electric current; acidic solutions therefore behave as electrolytes. Conductivity depends on acid strength (strong vs. weak) and concentration. For example, hydrochloric acid exhibits high conductivity, whereas weak acids such as acetic acid show lower conductivity.

Effect on Indicators

Acids cause color changes in pH-sensitive indicators, a feature used to identify acidic solutions and estimate pH qualitatively. Blue litmus paper turns red in acidic media. Phenolphthalein remains colorless in acidic solutions; methyl orange appears red in acidic conditions. Such indicators provide a visual means of detecting acidity.

Reaction with Metals

Acids react with active metals (e.g., zinc, magnesium, iron) to form salts and hydrogen gas (H₂). Because hydrogen is flammable, appropriate safety precautions are required during such experiments. Owing to their ability to dissolve metals, acidic solutions are generally not stored in metal containers; glass, plastic, or specially coated materials are preferred.

Effect on Carbonate Compounds

Acids react with compounds containing carbonate and bicarbonate ions to release carbon dioxide (CO₂). For example, contact between hydrochloric acid and calcium carbonate (CaCO₃) produces effervescence and surface etching. This reaction explains how acids dissolve carbonate minerals. The degradation of marble, limestone, or the calcium-carbonate skeletons of shellfish by acidic precipitation is governed by the same chemical principle.

pH

The acidity of a solution is measured by the pH scale. Solutions with pH values below 7 are acidic; decreasing pH (approaching 0) indicates increasing acidity. Strong acids (hydrochloric, sulfuric, nitric) have very low pH values, whereas weak acids (acetic, formic, citric) appear at higher pH values. The pH scale provides a universal metric for gauging the degree of acidity.

Strong Acids

Strong acids are substances that ionize almost completely in aqueous solution. Because the degree of ionization is close to 100%, the concentration of free hydrogen (H⁺) or hydronium (H₃O⁺) ions is high, leading to increased electrical conductivity and faster reaction rates. Typical properties include dissolving metals, rapidly decomposing carbonate compounds, and exhibiting corrosive effects on organic materials.

Notable strong acids include:

• Hydrochloric acid (HCl): Fully dissociates into H⁺ and Cl⁻ in water. Widely used in industry for metal pickling, in food processing, and in laboratories.
• Sulfuric acid (H₂SO₄): Very strong; its first dissociation is complete and the second is partial, yet it is treated as a strong acid in practice. Known for strong dehydrating ability. Extensively used in fertilizer production, batteries, and chemical manufacturing.
• Nitric acid (HNO₃): Fully dissociates in water. A strong oxidizer used in the manufacture of explosives, fertilizers, and dyes.

While essential in industrial processes, the high reactivity of these acids necessitates strict safety practices.

Weak Acids

Weak acids ionize only partially in water. Many molecules remain unionized, and an equilibrium forms between ionized and unionized species. Consequently, for solutions of equal concentration, weak acids display lower hydrogen-ion concentrations than strong acids, lower conductivity, and generally slower reaction rates. Examples include:

• Acetic acid (CH₃COOH): Present in vinegar; ionizes at roughly ~1% in household concentrations. Used widely in food and chemical industries.
• Citric acid (C₆H₈O₇): Naturally occurs in citrus fruits. Used as a food additive and preservative.
• Formic acid (HCOOH): Found in ant secretions and some plants; the simplest monocarboxylic acid with various biological roles.

Although less aggressive than strong acids, weak acids play important roles in biological systems and everyday applications.

The pH Scale

The most common tool for characterizing acidity or basicity is the pH scale. The term “pH” derives from “power of hydrogen” and is calculated as the negative logarithm of the hydronium-ion concentration:

pH = −log [H₃O⁺]

Typically expressed on a 0–14 scale, pH can fall outside these limits under special conditions (e.g., very strong solutions or different temperatures).

• pH < 7: Acidic. As pH approaches 0, [H₃O⁺] increases and acidity strengthens.
• pH = 7: Neutral. Pure water at 25 °C has a pH of approximately 7.
• pH > 7: Basic. As pH approaches 14, hydroxide-ion (OH⁻) concentration increases and basicity strengthens.

pH measurement is critical in biological systems (e.g., blood pH maintained at 7.35–7.45), agriculture (soil assessment), food technology, and environmental water-quality analysis.

Indicators

Indicators are commonly organic substances that change color depending on solution pH, allowing visual estimation of approximate pH.

• Litmus paper: Turns red in acidic media and blue in basic media.
• Phenolphthalein: Colorless below pH 7; pink between pH 8.2–10; deep pink-red in more strongly basic solutions.
• Methyl orange: Red in acidic solutions, yellow in basic; often used in titrations.
• Natural indicators: Plant-based substances can function as indicators. Red cabbage extract appears red–pink in acids and blue–green to yellow in bases; frequently used in educational experiments.

Neutralization Reactions

When an acid reacts with a base, they largely lose their characteristic properties and form a salt and water—a process termed neutralization:

Acid + Base → Salt + Water

Example: Reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH) yields sodium chloride (NaCl) and water (H₂O):

HCl + NaOH → NaCl + H₂O

The nature of the resulting salt depends on the strengths of the reactants:

• Strong acid + strong base → neutral salt (e.g., NaCl)
• Strong acid + weak base → acidic salt (e.g., NH₄Cl)
• Weak acid + strong base → basic salt (e.g., Na₂CO₃)

Neutralization is significant in biological contexts (e.g., buffering stomach acid with antacids) and industrial applications (e.g., pH control in wastewater treatment).

Common Acids and Their Features

• Hydrochloric acid (HCl): Naturally present in gastric juice and aids digestion. Used for metal surface cleaning and synthesis of chlorine compounds.
• Sulfuric acid (H₂SO₄): Historically known as “oil of vitriol.” A key industrial chemical used as battery electrolyte, in fertilizer production, and in petrochemicals.
• Nitric acid (HNO₃): Known as “aqua fortis.” A strong oxidizer used in fertilizers, explosives, dye manufacture, and metal processing.
• Acetic acid (CH₃COOH): The weak acid responsible for vinegar’s sour taste. Used as a preservative and flavoring in foods, and in the production of polymers, solvents, and acetate salts.
• Citric acid (C₆H₈O₇): Occurs naturally in citrus fruits. Used as a flavoring, acidulant, and preservative in foods and beverages, and widely in pharmaceutical and cosmetic products.

Areas of Use

Human Body

Acids are critical in biochemical processes. Gastric hydrochloric acid (HCl) facilitates protein digestion and helps eliminate harmful microorganisms. Amino acids are the building blocks of proteins and central to cellular metabolism. Fatty acids form part of lipids and function in energy storage. Nucleic acids (DNA and RNA) carry genetic information. Buffer systems such as the carbonic acid/bicarbonate pair maintain blood pH.

Foods

In food technology, acids function as flavoring and preservative agents. Acetic acid (CH₃COOH) in vinegar and citric acid (C₆H₈O₇) in citrus fruits impart sour taste and suppress microbial growth, extending shelf life. Phosphoric acid (H₃PO₄) and carbonic acid (H₂CO₃) in carbonated beverages contribute characteristic sharpness and acidity.

Medicine and Pharmacy

Many pharmaceuticals are acidic. Acetylsalicylic acid (aspirin) is widely used as an analgesic and antipyretic. Ascorbic acid (vitamin C) supports immune function and prevents scurvy. Salicylic acid is used in dermatology to treat acne and calluses.

Industry

Acids rank among the most widely used industrial chemicals. Sulfuric acid (H₂SO₄) is a principal feedstock for fertilizers, detergents, dyes, explosives, and battery electrolytes. Hydrochloric acid (HCl) is used for metal cleaning, rust removal, and synthesis of inorganic compounds. Nitric acid (HNO₃) plays major roles in fertilizer and explosive production and in metal processing.

Acid Rain

Acid rain forms when atmospheric pollutant gases react with water vapor to produce acidic solutions. Sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) released during fossil-fuel combustion react with water vapor and oxygen to yield strong acids such as sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These compounds reach the surface via rain, snow, or fog and affect the environment.

Effects

• Ecosystems: Lowers pH in aquatic environments, creating unfavorable conditions for fish and other organisms. In soils, alters nutrient availability and impairs plant uptake.
• Agriculture: Changes in soil pH can reduce yields; nutrients such as calcium, magnesium, and potassium may leach from soil.
• Structures and Cultural Heritage: Causes erosion and deterioration of carbonate stone (marble, limestone) and accelerates corrosion of metals.
• Human Health: Precursor gases SO₂ and NOₓ can exacerbate respiratory diseases, triggering conditions such as asthma and bronchitis.

Mitigation and Control

Prevention involves reducing fossil-fuel use, employing desulfurization systems, catalytic converters, and expanding renewable energy. Liming can be applied to lakes and rivers to help maintain pH balance.